The field of the present invention relates to a radio frequency (RF) coil and a magnetic resonance imaging apparatus using this RF coil.
A magnetic resonance imaging apparatus is an apparatus that generates magnetic resonance signals by utilizing the magnetic resonance phenomenon and obtains tomograms of a subject. A magnetic resonance imaging apparatus uses an RF coil for transmitting and receiving magnetic resonance signals. This RF coil is required to have a high signal to noise ratio (SNR) because it handles very weak signals.
In order to realize a high SNR, a small looped coil is used to enhance the sensitivity of the coil. However, a shrunken square measure makes it impossible to image a large area in one shot. For this reason, in order to enhance the sensitivity and at the same time to image a large area in one shot, a plurality of coil elements are arranged to constitute one RF coil, and the plurality of coil elements are used simultaneously. However, the simultaneous use of a plurality of coil elements gives rise to interference mainly by electromagnetic inductance among the coil elements. Known techniques to eliminate or reduce this interference include the following.
One of known such methods is to form an RF coil by arranging a plurality of coil elements in a partly overlapping way. FIG. 12 shows an example of RF coil 1. The RF coil 1 consists of an array of four loop-type coil elements 2. The RF coil 1 can obtain an image of a large area while keeping a level of sensitivity comparable to that of a small looped coil. By appropriately selecting the proportion of overlapping areas between adjoining coil elements, electromagnetic interference can be eliminated (see, for example, Japanese Unexamined Patent Publication No. Hei 11 (1999)-318851).
There is another method which uses a neutralizing circuit formed of a condenser bridge. This neutralizing circuit spans two coil elements by a condenser bridge, which supplies a voltage reverse in phase to an induced electromotive force generated by mutual inductance and thereby cancels electromagnetic interference (see, for example, Japanese Unexamined Patent Publication No. Hei 8 (1996)-229019).
There further is a method that uses a preamplifier of a low input impedance. By this method, an inductance linking the low-input impedance preamplifier and an RF coil and an output capacitor of the RF coil reduce the current of a Larmor frequency and can thereby reduce mutual inductance between the coil elements (see, for example, Japanese Unexamined Patent Publication No. 2001-327477).
However, the methods described above involve the following problems.
By the method of forming an RF coil by arranging a plurality of coil elements in a partly overlapping way, electromagnetic interference can be eliminated by appropriately selecting the overlapping areas between adjoining coil elements, but varying the overlapping areas between coil elements would give rise to electromagnetic interference. Therefore by this method, if the size of the RF coil is varied to match the region or the bodily shape of the subject to be imaged, the overlapping areas between coil elements will vary, resulting in coupling of the coil elements and a drop in SNR. If the RF coil is used without varying its size and the subject is small, the distance between the RF coil and the subject will become too large, inviting a drop in SNR and deterioration in the picture quality of the obtained image. If the subject is large, the imaging area in which the RF coil can shoot the subject will shrink, resulting in an increased number of required times of imaging.
Further, by the method which uses a neutralizing circuit formed of a condenser bridge, the need for the neutralizing circuit means a higher cost. Further, if the size of the RF coil is increased to obtain a wide diagnostic image in one shot, the number of coil elements constituting the RF coil will increase, and the increase in the number of coil elements will invite correspondingly more complex wiring.
Further, by the method using a preamplifier of low input impedance, the low-input impedance preamplifier means an extra cost as does the neutralizing circuit. Moreover, where the magnetic interference between coil elements is intense, the eliminating function may not prove sufficient.